1. Field of the Invention
The invention relates to a method for absolute preserved amplitude processing of data obtained by means of the seismic prospecting technique known as VSP, wherein seismic waves received by one or more multi-axis pickups coupled with the formations surrounding a well, coming from a seismic source arranged at the surface, either under direct arrival conditions, or after reflection on discontinuities of the underlying formation, are recorded.
2. Description of the Invention
The VSP technique is conventionally used to measure propagation times and velocities, and to obtain a zero-phase reference of the series of reflections on the reflectors encountered by the well (the stacking domain located immediately below the VSP measuring points is commonly referred to by specialists as corridor stack or VSP log, a designation that is used in the description hereunder). However, this series is produced by means of processing tools which modify the amplitude of the reflected signals: multiplication by a constant gain, rough spherical divergence compensation, dynamic time equalization and spectral equalization, etc. In fact, conventional methods allow recovery of the amplitude contrasts of the reflections in relation to one another according to the processing procedure used, but in practice they fall to recover the absolute amplitude ratio of the reflected waves in relation to the direct waves reaching the receivers. As a consequence, on the one hand, diffractions of very high amplitude may be mistaken for reflections, which leads the interpreter to be mistaken in the assessment of the structure in the vicinity of the well and, on the other hand, the real amplitude of the reflections cannot be exploited or interpreted.
The prior art in the field of seismic attenuation measurement, in particular by means of the Vertical Seismic Profiling method, and its consideration during processing, is illustrated by many publications, and notably by the following publications:
Gardner, G. H. F., L. W. Gardner, and A. R. Gregory: Formation Velocity and Density;
The Diagnostic Basics for Stratigraphic Traps. Geophysics Vol. 39, No. 6, 1974, pp. 770-780;
Hauge, P. S.: Measurements of Attenuation from Vertical Seismic Profiles, Geophysics, Vol. 46, 1981, pp. 1548-1558;
Kan, T. K., et al.: Attenuation Measurement from Vertical Seismic Profiling, SEG Expanded Abstracts, LA meeting, October 1981, pp. 338-350;
Lee, M. W., et al.: Computer Processing of Vertical Seismic Profile Data, Geophysics, Vol. 48, No. 3, March 1983, pp. 282-287;
Newman Paul: Divergence Effects in a Layered Earth, Geophysics, Vol. 38, No. 3, June 1973, pp. 481-488;
Newman, P. J., et al.: In Situ Investigation of Seismic Body Wave Attenuation in Heterogeneous Media, Geophysical Prospecting 30, pp. 377-400, 1982;
Payne, M. A.: Looking Ahead with Vertical Seismic Profiles, Geophysics Vol. 59, No. 8, August 1994, pp. 1182-1191;
Pujol et al.: Interpretation of a Vertical Seismic Profile Conducted in the Columbia Plateau Basalts, Geophysics, Vol. 54, No. 10 (October 1989), pp. 1258-1266;
Pujol and Smithson: Seismic Wave Attenuation in Volcanic Rocks from VSP Experiments, Geophysics, Vol. 56, No. 9 (September 1991), pp. 1441-1445;
Spencer T. W. et al.: Seismic Qxe2x80x94Stratigraphy or Dissipation, Geophysics, Vol. 47, No. 1 (January 1982), pp. 16-24;
Spencer, T. W., 1985: Measurements and Interpretation of Seismic Attenuation in Fitch, A. A., Ed. Developments in Geophysical Exploration Methods, 6, Elsevier Science Publ. Co. Inc., pp. 71-109;
Stainsby S. D. et al.: Q Estimation from Vertical Seismic Profile Data and Anomalous Variations in the Central North Sea, Geophysics, Vol. 50, No. 4 (April 1985), pp. 615-626;
Rainer Tonn: The Determination of the Seismic Quality Factor Q from VSP Data. A Comparison of Different Computational Methods, Geophysical Prospecting, April 1990;
Ross, W. S., et al.: Vertical Seismic Profile Reflectivity. Ups Over Downs, Geophysics, Vol. 52, No. 8 (August 1987), pp. 1149-1154;
Rutledge, J. T., and Winkler, H., Attenuation Measurements in Basalts Using Vertical Seismic Profile Data from the Eastern Norwegian Sea: SEG, Expanded Abstracts, pp. 711-713, New Orleans, 1987;
Sokora, W. L., 1996, Predicting Formation Target Depth Ahead of the Bit with High Accuracy: A case Study from the Arun Field for a Deviated Well: Proceedings of the indonesia Petroleum Association, IPA96-2.5-028;
Wu R. and K. Aki, Scattering Characteristics of Elastic Waves by an Elastic Heterogeneity, Geophysics, Vol. 50, No. 4, April 1985, pp. 582-595;
Yuehua Zeng, Feng Su and Keiiti Aki, Scattering Wave Energy Propagation in Random Isotropic Scattering Medium, JGR, Vol. 96, No. B1, pp. 607-619, January 1991.
The aforementioned publications describe methods of measuring the attenuation of seismic waves in transmission for vertical seismic profile data (VSP). These measurements are sometimes performed too roughly but, unfortunately, none of these publications provides a solution concerning the way to use these measurements so as to more exactly recover by processing the amplitude of the reflected events observed on the VSPs, for any distance between the position of the well pickups and of the reflectors, including reflectors located below the well bottom, which is the major object of the method according to the invention.
The spherical divergence, which is the most important factor in the amplitude decrease of a spherical seismic wave, is often compensated by an approximate law of Z=Vt type (Newman and Worthington, 1982), or by an exponential law of exp(xcfx80f xcfx84/Q) type for the events reflected below the well bottom (Payne, 1994), or by a rough time power law Tn, superscript n being adjusted by guesswork typically between 1 and 2, as it is generally done by well survey service companies. In a stratified medium close to a one-dimensional model, the spherical divergence can be taken into account more accurately by a t.V2 law (Newman, 1973), but this relation is rarely used (Pujol, 1991). The local impedance is never taken into account in the aforementioned publications, and the amplitude of the reflections is never examined. The 1D hypothesis is always made, but never verified in the literature. Many authors use a method of studying the evolution of the amplitude spectrum ratio of the direct arrival of the VSP taken at different depths (Kan, 1981) to determine the attenuation and the quality factor Q which characterizes it; others (for example Stainby, 1985) use the widening of the direct arrival pulse width: these methods may therefore be very sensitive to the reflected or diffracted waves that interfere with the direct arrival. Some authors, such as Rainer Tonn (1990), have successfully compared various measuring methods.
All the methods used assume the stationarity of the signal of the VSP downgoing wave, and this hypothesis is unfortunately not always verified in real cases. In effect, the fact that a spherical wave is propagated in a 1D stratified medium implies that part of the energy transmitted in P wave is converted to an S wave, even for low propagation incidences, and therefore the attenuation measured on the direct wave is often overestimated.
However, the order of magnitude of the measured attenuations is 1 to 13 dB per 1000 m (Pujol, 1989) for heterogeneous sedimentary or volcanic rocks.
The velocity variation function of the frequency is often insignificant between 10 and 100 Hz, even when considering a dispersive model of intrinsic attenuation, and the inner multiples can generate by themselves a not insignificant fraction of the total attenuation, for example 30% or 2 dB for 1000 m (Kan, 1981).
Any velocity heterogeneity close to the well can produce interferences which in most cases attenuate direct arrivals, but sometimes amplify them. This also depends on the way the amplitude is measured on the direct arrival (on the peak, the trough or the spectrum, therefore with a windowing and an amplitude variation linked with the apodization of the signal selected, etc.). Besides, propagation in a medium with a random velocity, therefore very heterogeneous in view of the velocity, is difficult to study, as shown by the complexity of publications by authors such as Wu (Wu et al., 1985) and Yuehua Zeng (Zeng, 1991).
In general, zero-offset profiles recorded in a stratified sedimentary medium show a very stable direct arrival signal, mainly interfered by the reflections on the sedimentary layer boundaries, and they are well-suited for fine study of the attenuation of the seismic signal.
The first two causes reminded hereafter relate to the constancy of the plane wave energy upon emission and reception, the two others concerning the effects due to the propagation of the seismic signal transmitted:
a) The amplitude variations of the source, which require recording of a reference signal. In practice, very repetitive sources as regards the form of the signal emitted are used for VSP acquisition, and a surface pickup arranged at a short distance from the source is sufficient to check the repetitiveness and to compensate variations in the emission energy and in the vertical stacking order.
b) The local impedance variations, which lead to variations in the amplitude of the plane wave transmitted at constant energy. In order to draw amplitude attenuation curves, the square root of the energy has to be represented, which amounts to saying that the amplitude observed at different points of a medium of variable acoustic impedance is brought back to the amplitude of a wave of equivalent energy in a single impedance medium. The interval velocity is given by the VSP and the density can be estimated by first approximation by Gardner""s law (Gardner, 1974) from the interval velocity.
c) The spherical divergence, which shows the expansion of the wavefront, depends for its compensation on the difference of the radii of curvature between two points located on the same seismic raypath. In particular, in the case of the VSP, compensation of the spherical divergence effect between the direct arrival and the reflections that follow is desired. This compensation must be very precise because the spherical divergence is the main attenuation factor, of an order of magnitude is greater than the cumulated other causes. This compensation depends on the source-geophone distance and on the characteristics of the depth interval between the geophone and the reflector below. It has the effect of bringing the amplitude of the wave emitted by a point source back to the amplitude of a plane wave for a direction of propagation identical to the direction observed at the measuring point, while disregarding the wave mode conversions (converted P-S or S-P).
d) The attenuation of a plane wave in transmission, all causes taken into account, in a 1-D medium, which includes, in a non limitative way: primary reflections, short-period inner multiple reflections, intrinsic transmission attenuation and diffusion/diffraction, etc., as far as these effects remain statistically 1-D as regards the roughness of the interfaces and the heterogeneity distribution.
This attenuation is calculated from the amplitudes of the VSP direct arrivals. It is identical, for a 1D medium, in two opposite propagation directions because of the reciprocity of the paths for a wave of a given type (of pure P or pure S type); under such conditions, for a vertical two-way path of a plane wave in a medium with homogeneous horizontal layers, and with a zero offset, the attenuation for a reflection is equal to the square of the attenuation measured on the first arrival for the corresponding one-way path. It is thus compensated in two-way propagation (loop travel) for the interval between the direct arrival and the reflections that follow, by multiplying the amplitudes of the reflections by the square of the inverse of the one-way path attenuation measured on the same depth interval. This correction does not depend on the source-geophone distance. The spherical divergence compensation and plane wave attenuation laws thus do not have the same form, which explains why it has always been difficult to compensate these two effects in combination by means of empirical laws. Furthermore, the use of the plane wave compensation outlined above allows determination of the accuracy limits of the attenuation calculations carried out on the direct arrival, and an empirical approach based on sound and reasonable geological and geophysical hypotheses allows, when the hypothesis of propagation in a one-dimensional medium is valid, to refine determination of the attenuation if the object of the operation.
The plane wave attenuation can depend on the frequency in any way and is determined by frequency bands.
The method according to the invention allows correct recovery of the absolute amplitudes of the events reflected through finer processing in order to obtain as the final product of VSP, on the one hand, in absolute amplitude, a quantified series of reflection coefficients encountered at the location of the well, and on the other hand, in preserved amplitude, i.e. with the highest accuracy possible on the amplitudes, a reflected wavefield that is referred to as deep when the distance between the downhole pickup and the impedance contrast generating a reflection is great, in particular below the well bottom, in order to make for example a more accurate prediction of the characteristics of the formations while drilling operations are in progress.
The processing method according to the invention allows recovery of the absolute amplitude ratios between, on the one hand, the seismic signals corresponding to upgoing waves emitted by a seismic source coupled with a geologic formation, then reflected on subsoil discontinuities, these signals being received by various seismic receivers coupled with the wall of a well through the formation and at a distance from one another, and on the other hand the seismic signals corresponding to downgoing waves (or direct arrivals) received by the same seismic receivers and coming directly from the seismic source.
In order to achieve this, the method determines quantitatively all the main causes of the attenuation of the seismic waves, in using them to compensate in the most suitable way the amplitude of the reflections measured by VSP according to the distance from the reflection point to the receivers in the well, and also in recovering the exact amplitude, referred to as absolute amplitude, of the coefficients of the reflections observed, in percentage, because this quantitative information has a specific incidence both for geologists regarding interpretation of the seismic prospecting results and for geophysicists carrying out surface seismic prospecting regarding the adjustment of certain acquisition parameters or the processing of surface seismic survey data. The method comprises the following steps:
a) First normalizing the direct arrivals at seismic receivers (R) in form of a zero-phase unit amplitude pulse in a limited frequency band determined by the signal-to-noise ratio observed in this band, which is carried out after deconvolution of the signature of the total field of the downgoing and upgoing waves by the downgoing waves, which allows compensation of the reflected arrivals for all the physical attenuation causes concerning the direct arrival path between the source and the receiver. This compensation includes, for example, all the possible amplitude and phase variations of the direct arrival and of the downgoing wavetrain according to the depth of the receiver, the spherical divergence of the direct arrival, and the effect due to the impedance of the geologic formation locally at right angles to the receiver,
b) then separating the upgoing and downgoing waves by means of multitrace velocity filters whose parameters are adjusted to the waves observed,
c) then compensating the differences between the amplitudes received by each seismic receiver, due to the spherical divergence between the paths of the upgoing waves and the paths of the downgoing waves coming directly from the seismic source. This compensation is preferably calculated univocally by the depth of the receiver, the depth of the underlying reflector and the velocity characteristics of the propagation medium.
According to an embodiment, the method comprises compensating the transmission attenuation (selectively by frequency band preferably) on the two-way path between the level of each seismic receiver and the level of each reflecting discontinuity, calculated from the amplitudes measured on the direct arrivals at the seismic receivers.
According to an embodiment, the method comprises compensating the transmission attenuation on the two-way path between the level of each seismic receiver and the level of each reflecting discontinuity so as to normalize the amplitude of key reflectors at the value measured on the receivers placed immediately above the key reflectors.
A key reflector normalization law allowing precise determination of the attenuation of the formations intersected in the depth zone is for example selected. The plane wave attenuation is thus measured in a single wave mode, either pressure wave (P waves) or shear wave (S waves), that is not affected by mode conversions and transmission losses.
According to an embodiment, the seismic energy lost by wave mode conversion during transmission through the reflecting seismic interfaces in the frequency band is determined by the difference between the two-way plane wave attenuation law used to normalize the amplitude of key reflectors in the depth zone (preferably in a structural environment comparable to a 1D one-dimensional model) and the square of the one-way plane wave attenuation law measured on the direct arrivals at the seismic receivers and in the same frequency band.
The method can comprise an impedance inversion of the stacked seismic trace (VSP log) or of any preserved-amplitude well survey image: preserved-amplitude reflected wave field, imaged by an offset VSP profile, by a deflected well VSP profile or by a walkaway type seismic well profile with a mobile source. This operation allows determination of the seismic impedance and velocity of the formations below the depth reached, for the time being, by the drilling operation and consequently to improve the efficiency of the decisions made for the continuation of the drilling operation. This specific application of the vertical or deflected well VSP, commonly referred to as xe2x80x9cprediction VSP beyond the bitxe2x80x9d or xe2x80x9cbeyond the hole bottomxe2x80x9d, is carried out either from a VSP called xe2x80x9cintermediatexe2x80x9d profile, recorded prior to laying an intermediate tubing, or from a VSP recorded during drilling and processed several times during hole deepening.
In cases where each seismic receiver comprises three pickups oriented along three different axes, the method comprises for example isotropic processing of the three oriented components and taking into account the total resultant of the downgoing direct wavetrains for the deconvolution and normalization operations.
The method can also comprise preprocessing so as to compensate the amplitude variations of the waves emitted by the source, due to repetitiveness defaults, and a signature deconvolution of the seismic source.
The method according to the invention does not involve analysis of the amplitude spectra of the direct arrivals, or any amplitude decrease law as a function of the frequency, but only the initial VSP measurements, by measuring time attributes and amplitudes of the direct arrivals.
The method can be applied for recovery of the seismic events reflected in converted P-S or S-P type mode, or in pure S-S mode.